In epitaxial growth, surfactants have proved to be effective in controlling the thin film microstructure, composition and morphology and, hence, to improve the thin film properties and device performance. Copel et al. in 1989 first used As as a surfactant in the growth of Si/Ge/Si(001) to suppress island formation [1]. Surfactant effects may affect crystal growth in various ways. For example, surfactants can change the growth mode. In addition to Copel's work [1], the growth mode of Ag on Ag(111) is also changed when Sb is used as a surfactant [2,3]. Additionally, surfactants can reduce interface roughness. For example, Bi as a surfactant reduces the surface roughness of InGaAs grown on GaAs substrates [4]. Moreover, interface alloy intermixing can be suppressed by surfactants. For example, H can suppress the interface intermixing of Ge(001) covered Si [5]. Furthermore, surfactants can be used to change the surface reconstruction and, hence, control the formation of various new ordered phases. For example, Sb is known to suppress Cu—Pt ordering in GaInP [6]. At higher concentrations, surfactants can change the surface reconstruction from (2×4) to (2×3)—for example, inducing a new ordered phase in InGaP [6]. Also, surfactants can affect the incorporation of dopants in semiconductors [7,8].
The surfactant effects listed above may be attributed to several physical mechanisms. Surfactants can change the growth thermodynamics by altering the surface energy. For example, surface As is known to lower the surface energy of the Si/Ge/Si system to suppress island formation [1]. In addition to changing the thermodynamics, surfactants can change the growth kinetics, such as surface diffusion [2] and the size of step-edge barriers [3]. For example, Sb as a surfactant has been shown to reduce the mobility of Ag adatoms. This results in a higher island density leading to a change of growth mode. Sb as a surfactant on Ag (111) or GaAs can also reduce the step edge barrier and promote smoother growth morphologies [3,9].
Obtaining high doping levels in high band gap materials has been a difficult problem for decades. This hinders high-level p-type doping in III-V materials such as phosphide and nitride semiconductors. This may be caused by several factors, including the limited solubility of acceptors, H passivation of acceptors, and high acceptor-hole binding energies [10,11]. An effective approach to achieving high p-type doping levels in GaInP, GaP, and GaAs employs the use of surfactants during organometallic vapor-phase epitaxy (OMVPE) growth [6-8]. For example, a recent study showed that Sb can be used to enhance the incorporation of dopants, such as Zn [7,8], and reduce unintentional impurities, such as C, S, and Si [8]. In addition to Sb, surface H was postulated to play a role in the doping process [7,8]. The enhanced Zn doping was speculated to be caused by kinetic and/or thermodynamic factors. The presence of Sb may increase the surface diffusion of Zn and allow more Zn to reach step edges and incorporate into the film [12]. Also, the neutral Zn—H complexes have a lower film doping energy than the isolated Zn [7]. However, there remains insufficient understanding of the underlying doping mechanisms associated with surfactants because it is impossible to directly observe the microscopic doping process.